1. Field of the Invention
The present invention relates to a communication system for communication between a plurality of non-telephone terminals each connected to a public telephone network, and a control center for controlling those non-telephone terminals.
2. Description of the Prior Art
A system of this type is constructed such that a telephone set and a non-telephone terminal such as a gas meter terminal, both provided in a house, are connected to the same telephone line through a terminal adapter unit, the switching of the connection therebetween being performed by the terminal adapter unit. The gas meter terminal, a labor-saving device which provides automated meter reading, transmits information through a public telephone network to a gas control center (hereinafter referred to as a control center). The transmitted information includes any data issued from the gas meter terminal, preset data stored in the same terminal and any alarm information issued from the gas meter terminal immediately after the detection of a gas leak. For this, the terminal adapter unit is operative with a communication system of the type where any signal is sent only from the terminal side.
Automated gas metering, however, requires some associated operations: In addition to the data transfer from the gas meter terminal to the control center as mentioned above, the gas meter terminal should be checked if any abnormal value is found in collected data. On/off control of a gas valve is desirable when the user is away from his house over a long time. Other desirable operations include checking on the conditions of gas usage at all times, resetting of the gas counter to zero, informing the user if there is a change in the telephone number of the control center, correction of the time indicated on a clock included in the terminal adapter, etc. To satisfy these requirements, two situations can be considered: In one situation, the terminal may be required by the control center to transfer additional data concerning those requirements by makinq use of a channel established therebetween when the terminal transmits ordinary data to the control center, and in the other situation, the terminal needs instruction from the control center. However, since in the conventional system described above the terminal adapter unit adopts the previously mentioned communication system, of the type where the terminal establishes a call, the control center cannot transmit such a signal concerning those requirements. That is, the convention system must rely upon some other informing means such as those mediating a telephone or a person engaged with that work and the like.
There is another known system, called the control center sending system, by which the control center can send such a signal at any time. Such a system is disclosed by NTT as "An Interface for No-Ringing," Communication Services (Proposal), Nippon Telegraphy and Telephony K.K., May 26, 1962. In this system, a local switch has the function of sending a terminal identifying signal. Once the control center calls an associated terminal, the local switch sends a call signal of a specific frequency to a terminal adapter. The terminal adapter identifies the call signal as designating the terminal, and connects the telephone line to the terminal.